Trimarans usually comprise a large central hull supported on each side by outrigging side hulls known as amahs. The outrigging side hulls are usually smaller in length and cross section than the main hull and have considerably less draft. The amahs are designed to provide buoyant support for a trimaran, especially at slow speeds and also to provide adequate stability. For a high speed vessel it is required that the amahs should be as small as possible to minimise drag and thus naval architects generally design trimarans with the amahs of minimal length and width consistent with the need for adequate stability.
The width of the amahs in way of the water must also be minimized in order to reduce the drag and reduce the generation of waves which may impact upon the main (centre) hull of the trimaran, thus causing additional drag. Typically the width of the amah in the water for a vessel having a length of about 120 metres may be as small as 700 millimeters at the top, tapering down to zero width at the bottom. Access for construction and maintenance is further substantially limited because of the internal stiffening structure.
The very narrow breadth of a trimaran amah makes the structure very narrow and introduces considerable problems in construction due to lack of access. The angle formed by the side plates at the bow of an amah in plan can be as low as 2° and usually less than 8° which obviously considerably constricts the available space.
It is also usual to reinforce the plating of hulls of this kind with longitudinally extending stiffeners which would become very closely packed together as the bow comes to a point and there is a need to terminate the stiffeners on a structure that is able to distribute the loads carried by the stiffeners.
It is these issues that have brought about the present invention.